Manufacture of coiled-coil filaments for electric incandescent lamps and similar devices



Patented June 8, 1937 PATENT ()FFICE 2,083,456 MANUFACTURE OF ,CO-ILED-COIL FILA- MENTS FOR ELECTRIC INCAN'DESCENT LAMPS AND SIMILAR DEVICES Reginald Edward Leeds, Middlesex, England, as-

signor to ration of New York General Electric Company, a corpo- No Drawing. Application January '7, 1937, Se-,

rial No. 119,527. 11, 1935 6 Claims.

This invention relates to the manufacture of coiled-coil filaments for electric incandescent lamps and similar devices.

Coiled-coil filaments are described in British patent specification No. 6,439 of 1914; but no detailed directions are given concerning their manufacture; in particular no heat treatment of the wire at any stage of the coiling is prescribed. In several later specifications heat treatment 10 after the winding of the secondary helix is prescribed at a temperature considerably exceeding 1000 C. Part at least of the object of this treatment is said to be the improvement of the stability of the filament, so that it will not sag seriously when maintained at the operating temperature. The increased stability is sometimes thought to be associated vwth the growth of long crystals. It appears to be believed very widely at the present time, that heating the secondary helix to a temperature considerably exceeding 1000 C. is essential to obtaining a stability sufficient for commercial purposes. So far as I am aware, such a heat treatment is applied in all I commercial processes for making coiled-coil filaments practiced at the present time.

I have. found that in suitable circumstances a stability of the filament sufficient for commercial lamps can be obtained without heating the secondary helix to any temperature above 1000 C. Sometimes the required stability can be obtained without any heating of the wire during coiling, that is to say at any stage between the cessation of Wire-drawing and the mounting of the coiled-coil filament on its final supports in the lamp. In other circumstances heat treatment may be necessary either before the secondary helix is wound or after it is wound or both; but it may still be possible to obtain the required stability without raising the temperature of the wire after the secondary helix is wound to above 1000f C. If the secondary helix has not to be heated, manufacture may be simplified and cheapened; thus it is not necessary to use the expensive molybdenum mandrels for the secondary-helix which are necessary if (as in some known processes) the secondary helix is heated on its mandrel to 1500 C.

In order that these results may two conditions must be fulfilled. The first is that the wire must be suitable. One of the necessary properties is ability to grow stable long crystals almost instantaneously when the completed filament is first heated in the lamp to or near to its operating temperature. Long crystals be achieved,

In Great Britain December means those whose length is not less than the 5 tainty all the necessary preparations or that set of them which is sufficient. It is well known that, in all filament manufacture, behavior in the'lampis the only safe criterion, and that the most careful description of the method of preparation or of tests to be applied (not simulating actual use) does not sufiice to ensure that all wires will be of the desired quality. ,On the other hand, wires that are suitable for the manufacture according to the invention may be expected to be found in the stock of any wellequipped maker of tungsten wire for filaments.

The methods of preparation which, in my experiments, have yielded suitable wire are (a) the Well-known modern silicated process, which is a development of that described in British patent specification No. 155,851, and (b) a process of subjecting a wire containing about three-fourths per cent of thoria to a thermo-mechanical process of the kind described in British patent specification No. 183,118. It can be stated definitely that no method of preparation known at the date of the aforesaid British patent specification No. 6,439 of 1914 will yield suitable wire.

Concerning the properties that the resulting wire should possess, it may be said that they are generally those recognized as favorable in the manufacture of coiled coil filaments by known methods; in particular, the greater the tendency of the wire to grow long crystals rapidly when coiled in the usual manner, the greater, in general, will be the tendency to grow long crystals rapidly when coiled according to the invention.

The second condition aforesaidis that the strains produced in the wire by the winding of the primary helix shall be at least partially released before the secondary helix is wound. These strainsmay be released, for example, by dissolving out-the primary mandrel or by heating the primary helix on a mandrel of molybdenum to a sufficiently high temperature, for example, 1600 C.

According to the invention, a process of manufacturing coiled-coil filaments for electric incandescent lamps comprises the following steps in the following order: (1) winding on a mandrel a tungsten wire capable of growinglong crystals almost instantaneously when first heated to the operating temperature, so as to produce a primary helix, (2) releasing part at least of the strain in the wire generated by the said winding,

' drel must be so small that a sumcient permanent ent 155,851 and known to V coil'filaments by known process. Q

i mm. in diameter.

' in diameter with a pitch of (3) winding the primary helix on amandrel so as to form a secondary helix, (4) removing the secondarymandrel, wherein the said wire, between the beginning of step (3) and the end of step (4); neverrises to a temperature greater than 1000 C.

I In steps (1) and (3) the diameter of. the manset shall be given to the wire. I may be encouraged in step (3) by heating the tungsten wire to'a temperature below 1000 C., e. g. 400 to 800 C. In suitable conditions a sufficient set may be'obtained even if the-diameter of the mandrel is as great as four timesthe di-" ameter of the wire in both steps. If in step (2) the strain is released by dissolving out the man- "drel, the primary helix must be subjected to a small suitably chosen tension instep (3).

Three processes according to the invention M will nowbe described byway of example. In all of them the tungsten wire, which serves as starting material, was a silicated'wire of the type described in the above-mentioned British Patgive excellent coiled- In the first process, the original wire was 0.031

primary mandrel: was dissolved out. The secondary helix :was formed bywinding this primary "helix in 137 turns on a mandrel again 0.12 mm.

' 0182mm .that ils'to say, with the turnstouching. Duringthis secondary windingthe primary helix was stretched so that its pitch became 0.0538 mm. The sec- 'ondary helixwas heated to 400 C. for two minutes to encourage apermanent set; after removal of the secondary mandrel the pitch of the secondaryhelix was 0.357 mm. and the number of turns 100.

The second process was generally similar, but

the dimensions were changed and there was no heating in step (3) referred to above. The original wire. was 0.024 mm. diameter; the primary the primary pitch mm. Thesecondary mandrel was 0.12 mm. in

diameter," the pitch of the primary helix fduring winding of thesecondary helix was0.0333 mm., the secondary pitch 0.093 mm. '(turnstouching), the 'numberof secondary'turns 217. After removal of the secondary mandrel, the secondary pitch was 0.196 mml and the number of turns 157. In the third process, the original wire was 0.024- mm. diameter, the primary molybdenum mandrel 0.045 mm., the primary pitch 0.040 mm.

Theprimary'helix was heated on the mandrel to .1500 Cffor, twelve seconds, but the'mandrel was notremoved The secondary mandrel was 0.210 mm.,diameter, the pitch 0.155 (turns not touch 1 mg), the number of turns 15 5. After-the windcompriseswinding on a mandrel a tungsten wire 1 ing of the secondary helix, the wire washeated to 750 CQfor thirty seconds. After'removal of the mandrels, the pitch of the secondary jhelix was 0.176 mm.; thenumber of turns 136.

What I claim as new and desire tov secure by Letters Patent of the United States is:

1. The process of "manufacturing coiled-coil which grows'long' crystals almost instantaneously when first heated to the operating temperature a A permanent set in the lamp, 7 7 It was coiled into, a helix mms. long with a pitch of 0.0396 mm. on amolyb- *denum mandrel 0.12 mm. in diameter; j This' temperature of between 400 C.'and 800 drel.

g a temperature maintained below 1000 C., and

then removing the secondary mandrel.

The process of manufacturing coiled-coil filaments for electric incandescent lamps which comprises winding on a mandrel a tungsten wire which grows long crystals almost instantaneously when first heated to the operating temperature in the lamp, so as to produce a-primary helix, removing saldmandrel to release at least part of the strain inthe wire produced by the winding, winding the primary helix on a secondary mandrel so as to form a secondary helix, heating the secondary helix-to a temperature maintained below 1000 C., and then removing the secondary mandrel. h

3. The process of manufacturing coiled-coil fil- "aments for electric incandescent lamps which comprises winding on a-mandrel a tungsten wire which grows long crystalsalmost instantaneously when first heated to the operating temperature so as to producea primaryhelix, heating the WIIe'tO a temperature of at least 1500f C. to' release at least part of the strainin the wire produced by the winding, winding the primary helix on a secondary mandrel so as to form a secondaryhelix, heating the secondary helix to a temperature maintained 'below'1000" 'C., and then removing the secondary mandrel.

4. The} process of manufacturing coiled-coil iilamentsfor electric incandescent lamps which comprises winding on a mandrel a tungsten wire which grows long crystals almost instantaneously when first heated to the Operating temperature inthe lamp, so as to produce a'primary helix, re-

leasing at' least part of the strain in thewire produced by the winding, winding the primary helix on'a secondary mandrel so as to form a secondary helix, heating the secondary helix to a C and then removing the secondarymandrel;

5. The process of manufacturing coiled-coil filaments for electricincandescent lamps which comprises winding on a mandrel a tungsten wire which grows long crystals almost instantaneously :when first heated to the operating temperatureimthe lamp,

so' as to produce a primary helix, removing said mandrel to release at least part of the strain in the wire produced by the winding, winding the primary helix on a secondary mandrel so as to form a secondary helix, heating the,

secondary'helix to a temperature of between 400 C. and 800C., and then removing the secondary mandrel.

'6. Theprocess of manufacturing coiled-coil gfilaments for electric incandescent lamps which 7 comprises winding on a mandrel a tungsten wire which grows long crystals almost instantaneously when first heated tothe operating temperature in the lamp, so as to produce a primary helix, heating the .wire .toja temperature. ofat least:

1500 'C..torelease at least part of the strain in the wire produced by the winding, winding the primary helix on a secondary mandrel so asto "form a secondary helix, heating the secondary helix toa temperatureof between "400 C. and 800 C., and then removing the secondary mananemam EDWARD LE'E'DS. 

